Facile fabrication of carboxymethyl cellulose sodium/graphene oxide hydrogel microparticles for water purification

Abstract

Herein, carboxymethyl cellulose sodium (CMCNa)/graphene oxide (GO) hydrogel microparticles (CGs) with diameters of 2.2–3.6 μm were prepared facilely via spray drying. The as-prepared CGs were used as adsorbents for the removal of methylene blue (MB), Eosin Y and various heavy metal ions. The results revealed that the adsorption capacities of the CG with 8 wt% GO increased by 340–354% for the dyes and 350–500% for heavy metal ions, in comparison with the pristine CMCNa reference. Moreover, the microparticle structure also endowed the CGs with high adsorption kinetics as the adsorption equilibrium for all pollutants was commonly reached within 30 min. The adsorption mechanisms for dyes were due to both electrostatic and π–π interactions, while those for heavy metals were the synergistic effect of electrostatic interactions, surface complexation and ion exchange. Further investigation indicated that the adsorption isotherms and kinetics can be well fitted by the Langmuir isotherm model and the pseudo-second-order model, respectively. By adjusting the GO content and solution pH, the adsorption capacities of our CGs can be further improved. The unique adsorption properties of our CGs, together with the spray drying method, make them very promising in the mass production of broad-spectrum adsorbents for water purification.

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1. Introduction

The rapid development of petrochemical industries associated with textiles, leather, paper-making and printing, has generated an enormous amount of wastewater that contains toxic dyes, heavy metals and aromatic pollutants. Consequently, a variety of techniques including adsorption, membrane separation, electrocoagulation and so forth, have developed gradually for the removal of the above industrial contaminants.^1–3 Due to its low cost, ease of operation and fewer harmful secondary products, adsorption is thought to be the most effective and widely used method. To date, different absorbents, such as activated carbon, wood chip, fly ash and coal, synthetic polymer resin and polysaccharide-based materials, have been prepared for water purification.^4–6 However, their drawbacks of low adsorption capacities and efficiencies hardly meet the demands of increasingly serious environmental problems.

Hydrogels, one sort of hydrophilic polymers connected by chemical and/or physical crosslinking,⁷ have 3D network structures and large specific surface area, and thereby are usually employed as superior adsorbents. In the fabrication of such materials, biopolymers are more attractive than synthetic ones, because of their superior biocompatibility and biodegradation.⁸ As the most abundant nature polymer, cellulose, as well as its derivatives like carboxymethyl cellulose (CMC), have been made into hydrogels for wastewater treatment.^9–12 But their adsorption performances are too weak, especially for heavy metals. To further improve the decontamination capacities of hydrogel materials, functional nanofillers began to be introduced into their networks.^13–18

Graphene, a new 2D material with sp²-conjugated carbon nanostructure and high specific surface area,¹⁹ is able to capture aromatic pollutants via π–π interactions.^20–22 Its derivative, graphene oxide (GO), has plenty of oxygen-containing functional groups including epoxy, hydroxyl and carboxyl groups, and shows exceptional adsorption capacities for dyes, heavy metals, organic solvents and oils.^23–32 When incorporated into biopolymer-based hydrogels, these carbon fillers can either improve their adsorption ability towards a certain effluent,^33–35 or broaden the categories of pollutants.^13,36 For example, Chen et al. prepared chitosan (CS)/GO hybrid hydrogels as broad-spectrum adsorbents.³⁷ In aqueous solution, the negatively charged GO is capable of removing cationic methylene blue (MB) via electrostatic interactions, while the positively charged CS is of the adsorption of anionic Eosin Y. As a result, the as-prepared CS/GO hydrogels showed a high adsorption capacity (>300 mg g⁻¹) for both MB and Eosin Y. Moreover, the adsorption capacities of the hydrogels for Cu²⁺ and Pb²⁺ were found to be 70 and 90 mg g⁻¹, respectively. According to other reports,^10,24,37,38 the underlying mechanisms for the adsorption of heavy metals by CS/GO composites may be assigned to the combination of electrostatic interactions, surface complexation and ion exchange. On the other hand, these nanofillers can be utilized as one sort of reinforcing agents to enhance the mechanical properties of cellulose-based composite.^39,40 For instance, Zhang et al. fabricated orderly porous CMC/GO monoliths by an unidirectional freeze-drying method.⁴¹ The results revealed that the incorporation of GO not only improved the adsorption capacities of CMC hydrogels for heavy metals, but also significantly enhanced their compression strength. When the content of GO was 5 wt%, the compression strength of the composite reached to ∼41 kPa, much higher than that (∼17 kPa) of the pristine CMC in control. Recently, graphene has also been filled into cellulose and CMC hosts as conductive filler for energy storage and shielding the undesired electromagnetic interference.^42,43

Although great progress has been made, there are still some challenges. For the above CS/GO hydrogels, the time required for their adsorption equilibrium are about 58 and 36 h for MB and Eosin Y,³⁷ indicating slow adsorption kinetics. Similar phenomena are also observed in graphene hydrogels and porous CS/gelation/GO monoliths.^21,44 One of the possible reasons is believed to be their large volume, which hinders the diffusion of the adsorbate molecules through the hydrogels. Hydrogel microparticles (or microgels) have small volume and large specific surface area,⁴⁵ and may be a good option for the preparation of high-efficiency adsorbents. In previous report,⁴⁶ our group reviewed the recent progress in graphene-based hydrogels as well as their wide applications in water purification, supercapacitors, drug delivery, microfluidic switch, catalyst support and self-healing materials. To best of our knowledge, the adsorption behaviors of cellulose/GO or graphene composite hydrogel microparticles for organic dyes and heavy metals still remains unknown. Inspired by these, it is interesting to fabricate CMC/GO hydrogel microparticles and investigate their adsorption behaviors for industrial pollutants.

In this article, CMC/GO hydrogel microparticles (CGs) were prepared simply by spray drying. The structure of microparticle was expected to accelerate the rapid adsorption of pollutants, while the spray drying method may help to realize the mass production of CGs. As a result, the adsorption properties of the CGs for cationic MB, anionic Eosin Y and heavy metals ions were investigated under different conditions: contact time, GO contents and pH, and their adsorption mechanisms were also discussed. Furthermore, the corresponding adsorption isotherm and kinetics were analyzed to better understand the adsorption process.

2. Experimental

2.1. Materials

Graphite oxide was synthesized by a modified Hummers method,⁴⁷ from natural graphite powders provided by Qingdao Huatai lubricant sealing S&T Co. Ltd., China. Carboxymethyl cellulose sodium (CMCNa), citric acid (CA), MB and Eosin Y were purchased from Sinopharm Chemical Reagent, China. The stock solutions containing metal ions of Pb²⁺, Cd²⁺, Cu²⁺ and Cr⁶⁺ (1 mg mL⁻¹) were obtained from Central Iron and Steel Research Institute, Beijing. All the chemicals were used as received without further purification.

2.2. Preparation of CMCNa/GO mixtures via solution blending

CGs were fabricated by the combination of solution blending and spray drying methods as shown in Fig. 1. Prior to the spray drying process, homogeneous CMCNa/GO mixtures need to be prepared via simple solution blending. In a typical procedure, graphite oxide (0.50 g) was dispersed in distilled water (250 mL), and fully exfoliated into graphene oxide (GO) with the assistance of ultrasonication for 1 h. On the other hand, CMCNa (1.25 g) was dissolved in distilled water (250 mL) at 60 °C to obtain an aqueous solution. After cooling to room temperature, the aqueous solution was blended with GO suspension under vigorous stirring to form homogeneous mixture. Meanwhile, the cross linker (0.5 g), CA, was added. Similarly, the CMCNa solution with GO content of 0, 2, 4, 6 and 8 wt% were prepared and ready for spray drying.

Fig. 1 Schematic illustration for the fabrication process of CGs.

2.3. Fabrication of CGs via spray drying

In this process, CGs were fabricated using a mini spray dryer (BUCHI B-290). To better understand its working principle, a schematic diagram for the apparatus has been given in Fig. 2. As seen, CMCNa/CA/GO aqueous solutions were initially brought to an atomization device in flowing compressed hot air. After spray drying, the solutions were converted into powders that would be further transferred into a sample collector. The resulting powders were then transferred into vacuum oven and dried at 90 °C for 6 h, in order to guarantee the adequate cross-linking of CMCNa. The resultant microparticles are termed as CGs, where the subscript “s” represents the mass fraction of GO. For example, CG2 is the CMCNa hybrid microparticle with 2 wt% GO, while CG0 represents pristine CMCNa microparticle.

Fig. 2 Schematic diagram for the working principle of mini spray dryer.

2.4. Swelling experiments

The swelling experiment was performed by immersing dried CGs into distilled water at room temperature for 4 h. For the evaluation of swelling ratio (SR), the hydrogels needed to be separated by the centrifugation at 4000 rpm for 15 min, and then were weighted after removing the residual water with filter papers. The SR was calculated using the equation as follows:where m₀ and m₁ represent the weight of wet and dry hydrogels, respectively.

2.5. Adsorption experiments

All adsorption experiments were performed in sealed 50 mL glass conical bottles that contained 0.03 g of CGs and 25 mL of standard solutions. The model pollutant in the standard solutions involves both organic dyes (MB and Eosin Y) and heavy metal ions (Pb²⁺, Cu²⁺, Cd²⁺ and Cr⁶⁺), and their concentration were precisely tuned to 100 mg L⁻¹ with deionized water. When the experiments began, the bottles needed to be placed in a shaking water bath at room temperature.

The adsorption isotherm and kinetic studies for all pollutants was carried out on a representative sample CG8 under the same solution conditions (20 °C for all the model pollutants; pH = 6 for MB and heavy metals, and pH = 2 for Eosin Y). The concentration of dye solutions was measured with UV-vis absorbance, while that of heavy metals was determined by atomic absorption spectrometer.

The effect of GO content on the adsorption of MB, Eosin Y and the metal ions were investigated, respectively. Afterwards, CG8 was selected as a typical sample and the effect of the solution pH on its adsorption capacity was studied in the pH range of 2–12 for the pollutants. Meanwhile, the solution pH was adjusted with HCl and NaOH solution (0.1 mol L⁻¹).

2.6. Characterization

The Fourier transform infrared (FT-IR) measurements were recorded in KBr Pellets using a Thermo Nicolet 6700 spectrometer. The X-ray diffraction (XRD) was conducted using a Bruker AXS X-ray diffractometer at 40 kV and 40 mA for monochromatized Cu Kα (λ = 0.154178 nm) radiation, and the interlayer spacing (d₀₀₂) was calculated by Bragg's law: d = λ/2 sin θ.⁴⁸ The Raman spectra were monitored on a Labram spectrometer with a laser of 633 nm. Additionally, the morphology of CGs was also observed with a Hitachi S-4800 filed emission scanning electron microscopy (SEM) at the accelerating voltage of 8 kV. Before the observation, all samples needed to be coated with platinum. Ultraviolet-visible (UV-vis) absorption spectra were recorded with a Shimadzu UV-2401PC spectrophotometer.

3. Results and discussion

3.1. Chemical changes in the fabrication of CGs

In the fabrication of cellulose-based hybrid hydrogels, chemical cross-linking agents in the forms of sodium polyacrylates and formaldehyde compounds are commonly utilized to initialize the cross-linked reactions. However, the fact is that most of the chemical cross-linkers like divinylsulfone (DVS) are highly toxics that may cause significant environmental concerns.⁴⁹ As to the physical method by electron beam, it generally requires relatively high doses of irradiation and poses a risk of the degradation of CMC.⁵⁰ Therefore, a green cross-linker, CA, was developed for the cross-linking of CMCNa,⁵¹ and a heat treatment over 80 °C was generally required for the process. Based on these, CA was selected as a cross-linker in the present study and the chemical changes in CGs were investigated with FT-IR spectra before and after their fabrication.

As shown in Fig. 3a, neat CMCNa presents a characteristic peak at 1595 cm⁻¹, due to the stretching vibration of carboxylate group. Moreover, strong absorption bands are also observed at 3402 cm⁻¹ (O–H stretching vibration), 2912 cm⁻¹ (C–H stretching vibration), 1421 cm⁻¹ (–CH₂ scissoring vibration), 1322 cm⁻¹ (–OH bending vibration), 1060 cm⁻¹ ([double bond splayed left]CH–O–CH₂ stretching vibrations), respectively.⁴⁰ When the sample is heat-treated in the presence of CA, a new peak will appear at 1732 cm⁻¹, which is assigned to the stretching vibration of carbonyl group, suggesting the formation of anhydride (an intermediate necessary for the esterification reaction between CA and the hydroxyl groups of CMCNa).⁵¹ After the incorporation of GO, the absorption band around 1732 cm⁻¹ can still be observed (Fig. 3b), further confirming the successful cross-linking of the CMCNa. Moreover, the carbonyl stretch of the carboxylic groups of GO was overlapped with that of carboxylate groups of CMCNa at about 1595 cm⁻¹, indicating the strong interaction between the carboxyl groups of GO and the hydroxyl groups of CMCNa.³⁹

Fig. 3 (a) FT-IR spectra of CMCNa and CG0; (b) FT-IR spectra of CGs; (c) XRD patterns of graphite, GO and CG8; (d) Raman spectra of graphite, GO and CG8.

Aside from the cross-linking of CMCNa, the heat treatment process also brings about the partially reduction of GO, as proved by the XRD diffraction pattern shown in Fig. 3c. The interlayer spacing (d₀₀₂) of CG8 was calculated using Bragg's law to be about 0.55 nm, corresponding to the diffraction peak at 16.1°. This value is much lower than the 0.89 nm for GO (2θ = 10.3°), while slightly higher than the 0.34 nm for graphite (2θ = 26.6°), indicating that the embedded GO has been partially reduced.²⁵ Moreover, the Raman spectra in Fig. 3d reveal that the G-band of CG8 locates in the domain of 1579–1604 cm⁻¹, the peak position of graphite and GO, respectively. And the intensity ratio of the D-band to G-band (I_D/I_G), inverse to the average size of sp²-conjugated domains in graphene,⁴⁸ also exhibited an increasing tendency from graphite, to CG8 and GO, further affirming the partial reduction of GO.

3.2. Morphology of CGs

Fig. 4 gives the microstructure of CGs. As seen, CG0 is one sort of round or oval gel which owns an average diameter of 2.2 μm. In the heat-treated process, pronounced shrinkage occurred, resulting in the rough surface of CG0. Moreover, tenuous cracks are also observed obviously on its surface, due to the weak mechanical properties. After the addition of GO, the size of the particle increases to about 3.6 μm, and the shrinkage of the samples are largely suppressed because of the rigid structure of GO. More importantly, no obvious cracks are found on the surface of the CG8, possibly because of the reinforcement effect of GO on CMCNa matrix.^39,40

Fig. 4 SEM images of CG0 and CG8.

3.3. Swelling behaviors of CGs

As we know, swelling properties are critical to hydrogel materials. The swelling ratios of our CGs are shown in Fig. 5. It is found that all samples exhibit high swelling rates within the initial 30 min, and then little change is found in their values with time prolonging. As a result, the equilibrium swelling ratios exhibit an increasing tendency with GO content increasing. Similar swelling behaviors were also reported in poly(N-isopropylacrylamide)/GO and poly(vinyl alcohol)/GO composite hydrogels,^52,53 where the swelling ratio increment for composite hydrogels were ascribed to the hydrogen bond between GO and water molecules that is not existent in the pure polymeric hydrogel systems. Furthermore, they also pointed out that excessive GO content could cause the predominance of the hydrogen bond between GO and the polymers that have a competitive relationship with the bond between GO and water molecules, consequently leading to the increase in the cross-linking density and some decrease in the swelling ratio. Based on these information, we think that the swelling properties of our CGs may be possibly enhanced by the hydrogen bond between GO and water molecules. Generally, the more the GO content, the more obvious enhancement effect in the swelling ratio. At the same time, the hydrogen bond between GO and CMCNa proved by El Achaby, et al. and Yadav et al. in the studies on CMC/GO composite films,^39,40 should not be neglected. The competition in the hydrogen bonds between GO with water molecules and GO with CMCNa may result in the increase in the swell ratio, but the decrease in the increasing rate.

Fig. 5 Influence of GO content on the swelling ratio of CGs.

3.4. Adsorption of dyes

The adsorption properties of CGs for cationic MB and anionic Eosin Y were investigated. In Fig. 6a, the pure MB (pH = 6) and Eosin Y (pH = 2) aqueous solutions are in blue and orange colors, respectively. With the addition of CGs, the colors fade by degrees, suggesting the elimination of the dissolved dyes. Moreover, the sample with more GO possesses much higher adsorption capacity. The UV-vis absorption spectra in Fig. 6b and c further confirm this observation, for the characteristic absorption intensity of MB at 664 nm and Eosin Y at 516 nm decreases noticeably with GO content increasing. Consequently, over 95% dyes were captured by CG8 within 1 h. To better understand the adsorption process of the dyes, the adsorption isotherms and kinetics of CGs were researched using CG8 as a representative sample.

Fig. 6 (a) Digital images of the adsorption of MB and Eosin Y; (b) UV-vis spectra of MB solutions treated with CGs; (c) UV-vis spectra of Eosin Y solution treated with CGs; (d) adsorption isotherms of dyes on CG8; (e) plots of C_e/q_e versus C_e for the simulation of Langmuir model; (f) adsorption kinetics of dyes on CG8; (g) plots of t/q_t versus t for the simulation of pseudo-second-order model; (h) effect of GO content on the adsorption of dyes; (i) effect of pH on the adsorption of dyes.

3.4.1. Adsorption isotherms. The adsorption isotherms are usually utilized to analyze the interactions between the adsorbates and adsorbents. As shown in Fig. 6d, the adsorption isotherms of CG8 were analyzed when the equilibrium was reached. Here need to mention, the adsorption capacity of CG8 can be calculated by the following equation:²⁰where C₀ and C_e are the initial and equilibrium concentrations of adsorbate solutions (mg L⁻¹), respectively; V is the volume of the solution (L); m is the mass of the adsorbent (g).

Fig. 6d indicates that the adsorption capacity of CG8 increases sharply with their equilibrium concentration of dyes, and then starts to level off. The increasing driving force derived from the concentration gradient of the dyes is usually considered to be a reasonable explanation,²⁰ which accelerates the diffusion of the dyes on CG8. To simulate the experimental data, Langmuir and Freundlich models are the most commonly used two.

The Langmuir isotherm model is suitable for the monolayer adsorption occurring on a homogeneous surface where no subsequent interaction between the adsorbed species occurs. The equation can be expressed as:²⁰

where C_e is the equilibrium concentration of the dyes (mg L⁻¹); q_e represents the equilibrium adsorption capacity of CG8 (mg g⁻¹); q_m is the maximum adsorption capacity of CG8 (mg g⁻¹); K_L is the Langmuir constant (L mg⁻¹), which is related to the affinity of the binding sites. Generally, the values of q_m can be achieved from the slope and intercept of the linear plot of C_e/q_e against C_e as shown in Fig. 6e.

As to the Freundlich model, it is an empirical model based on multilayer adsorption on heterogeneous surfaces, and can be described using the equation as follows:²⁰

where C_e and q_e are defined as the same with those in the Langmuir isotherm; K_F and n are the Freundlich constants that associated with adsorption capacity and adsorption strength, respectively. Specifically, n stands for the favorability of adsorption and the degree of heterogeneity of the surface. For example, a high value (n > 1) suggests a favorable adsorption. As the above mentioned, K_F and n can also be calculated from the intercept and slope of the linear plot of log q_e versus log C_e.

The isotherm parameters of both models are summarized in Table 1. By comparing the values of the correlation coefficient R², we believed that the Langmuir model fits the adsorption data better than the Freundlich does. It demonstrates that the adsorption of MB and Eosin Y occurs in a monolayer manner.

Table 1 Isotherm parameters for the adsorption of dyes and heavy metal ions by CGs

Isotherm models	Parameters	Dyes		Metal ions
		MB	Eosin Y	Pb^2+	Cd^2+	Cu^2+	Cr^6+
Langmuir	KL	5.4	3.6	0.3	0.4	1.3	0.6
	qm	58.24	65.70	19.19	27.01	28.41	27.81
	R^2	0.9815	0.9837	0.9858	0.9944	0.9626	0.9846
Freundlich	KF	29.17	33.50	5.85	10.98	11.75	9.87
	n	3.63	3.19	3.32	4.08	3.25	3.34
	R^2	0.8729	0.8630	0.9075	0.6689	0.9120	0.9064

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3.4.2. Adsorption kinetics. The adsorption kinetics were also studied in the present work. Fig. 6f presents the time-dependent adsorption curves of MB and Eosin Y on CG8, in which the adsorption capacities of CG8 for the both dyes increases dramatically within the initial 30 min, and then gradually approaches to two different constants (about 59 mg g⁻¹ for MB and 66 mg g⁻¹ for Eosin Y) with the contact time increasing. These values are much higher than those of the CG0 in control (13 mg g⁻¹ for MB and 15 mg g⁻¹ for Eosin Y), indicating the increment of 354% for MB and 340% for Eosin Y. The reason can be possibly due to the enhanced interactions between the dyes and GO, which include both the electrostatic interactions and π–π interactions.^21,37 Moreover, the time required by our CS8 to reach the equilibrium is only about 30 min, much lower than that needed by CS/GO hydrogel monoliths (36–58 h),³⁷ indicating the high diffusion rates of the dyes among hydrogel microparticles. To further investigate the mechanism of the adsorption process, two typical kinetic models were introduced, thus pseudo-first-order and pseudo-second-order models.

The pseudo-first-order model can be expressed as:⁵⁴

Where q_e and q_t (mg g⁻¹) are the adsorbed contaminants on CGs at equilibrium and various times t (min), respectively. k₁ (min⁻¹) is the kinetic rate constant of the pseudo-first-order model of adsorption. In fact, the values of q_e and k₁ can be obtained from the intercept and slope of the linear plot of log(q_e − q_t) versus t.

The pseudo-second-order model contains different kinds of adsorptions including external film diffusion, adsorption and internal particle diffusion. Its equation can be given as:²¹

where q_e and q_t have the same definitions and units with those in the pseudo-first-order model. k₂ (g mg⁻¹ min⁻¹) represents the rate constant of the pseudo-second-order model of adsorption. This kinetic model assumes chemisorption to be the rate determining step.³⁸ Similarly, the plot of t/q_t versus t may also yield the values of q_e and k₂ in Fig. 6g.

After simulation, all parameters are listed in Table 2. Based on the correlation coefficient R² of the both models, it is evident that the pseudo-second-order model is more suitable for the adsorption by our CGs, further confirming that the adsorption of the dyes is dominated by chemisorption.

Table 2 Kinetic parameters for the adsorption of dyes and heavy metal ions by CGs

Kinetic models	Parameters	Dyes		Heavy metal ions
		MB	Eosin Y	Pb^2+	Cd^2+	Cu^2+	Cr^6+
Pseudo-first-order	k1 × 10^−2	10.09	10.57	4.12	5.09	5.57	5.14
	R^2	0.9967	0.9850	0.9150	0.8807	0.8643	0.8755
Pseudo-second-order	k2 × 10^−3	4.10	3.55	7.18	3.90	2.29	3.91
	R^2	0.9999	0.9978	0.9984	0.9882	0.9719	0.9825

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3.4.3. Effect of GO content and pH. As observed in Fig. 6a–c, the content of GO have a significant impact on the adsorption capacities of CGs. Therefore, Fig. 6h shows the plot of adsorption capacity against GO content. Apparently, the adsorption abilities of CGs can be enhanced by increased GO content. The main reason may be ascribed to the negatively charged carboxyl groups in GO sheets as well as its unoxidized sp²-conjugated areas as proved in Fig. 3c–d. The former can help CGs to capture more cationic MB and anionic Eosin Y via electrostatic interacts under different pH conditions, while the latter can also enhance the adsorption of these aromatic dyes via π–π interactions.^21,37

Additionally, the pH of the solution is another controlling factor that may exert a critical influence on the adsorption process. Therefore, the effect of pH on the adsorption of both dyes was studied over the pH range of 2–12. As shown in Fig. 6i, the adsorption curve of MB increases firstly and then declines. The adsorption of Eosin Y is quite different, the curve shows a decreasing tendency with the increase in the solution pH. It has been said that the pK_a of carboxylic groups is around 4.6.⁵⁵ Below this value, the –COO⁻ groups in CMCNa and GO will be protonated to –COOH groups, which makes CGs almost uncharged and benefits the adsorption of cationic MB and anionic Eosin Y via electrostatic attraction. When the pH increases above 4.6, ionization occurs to the –COOH groups, yielding the negatively charged CGs. Electrostatic repulsion will be the predominated force between Eosin Y and CGs, while for MB and CGs, electrostatic attraction will be enhanced. As a result, the maximum adsorption capacity of MB is obtained at pH = 6. Beyond this value, a screening effect will be formed by the excessive produced Na⁺,⁵⁵ causing the decrease in the adsorption of MB.

3.5. Adsorption of heavy metals

The adsorption of heavy metals was performed by immersing CGs in the desired standard solutions (100 mg L⁻¹) that were prepared by diluting the stork solutions with distilled water. Similar to the organic dyes, the analysis on the adsorption of heavy metals is also composed of three sections: adsorption isotherms, kinetics and effect of GO content and pH.

3.5.1. Adsorption isotherms. Fig. 7a gives the adsorption isotherm of CG8 towards heavy metal ions (pH = 6). For each metal ion, similar increasing tendency is observed with the increase in the equilibrium concentration. Moreover, for the samples with the same ionic valence number, the values of q_e decrease with the atomic number increasing, thus Pb²⁺ < Cd²⁺ < Cu²⁺.

Fig. 7 (a) Adsorption isotherms of heavy metal ions on CG8; (b) plots of C_e/q_e versus C_e for the simulation of Langmuir model; (c) adsorption kinetics of metal ions on CG8; (d) plots of t/q_t versus t for the simulation of pseudo-second-order model; (e) effect of GO content on the adsorption of heavy metals; (f) effect of pH on the adsorption of heavy metals.

When simulated with the two isotherm models, it is found that the adsorption behaviors of the metals are better fitted by the Langmuir model. And Fig. 7b gives the linear relationship of t/q_t versus t. The corresponding correlation coefficients R² in Table 1 suggest that the whole adsorption process of the metal ions is a kind of monolayer adsorption. This result is similar to the adsorption by GO sheets.^29,30

3.5.2. Adsorption kinetics. Fig. 7c presents the adsorption kinetics of CG8 for the metals. It is found that the amount of heavy metals reaches to high values within the initial 30 min, and then the increasing rates slow down. This phenomenon is similar to that observed in the above organic dyes. More importantly, the time of 30 min is extremely lower than those for CS/GO hydrogels (4–10 h),³⁷ and cellulose/chitin beads (4–5 h),¹⁰ which highlights the higher adsorption kinetics of our CGs. Additionally, the saturated adsorption capacity of CG8 for the metals with the same ionic valence number also tends to decrease with the atomic number increasing. The reason may be related to the radius of the ions. According to a previous report,¹⁰ a smaller radius means a faster adsorption process. For Pb²⁺, Cd²⁺ and Cu²⁺, their radius decrease gradually from 0.120 and 0.072 nm. Therefore, an increasing tendency is observed in the kinetics of these ions.

Moreover, the simulation using the pseudo-first-order and pseudo-second-order equations demonstrate that the latter is found to be more suitable for the adsorption behaviors of our CGs for heavy metals. The associated parameters are given in Table 2 and the linear plot for the calculation of the pseudo-second-order model is also shown in Fig. 7d. All these results indicated the chemisorption-dominated process.

3.5.3. Effect of GO content and solution pH. The effect of GO content on the adsorption abilities of CGs is shown in Fig. 7e. As expected, the adsorption of heavy metals is highly dependent on the content of GO. Generally, the more the GO content, the higher the adsorption capacity. Compared the negatively charged –COO⁻ groups of CMCNa and GO should be responsible for the enhanced adsorption of positively charged metal ions via electrostatic interactions, surface complexation and ion exchange.^10,24,37,44

As to the effect of pH, it is well known to us that the existence form of metal ions in aqueous solution can be significantly affected by the pH value. The reaction scheme for the hydroxide formation of divalent metal ions (Me²⁺) can be expressed as follows:²⁹

Me²⁺ ↔ Me(OH)⁺ ↔ Me(OH)⁰₂ ↔ Me(OH)₃⁻ ↔⋯

Previous report demonstrated that, at low pH < 6, the predominant metal species in aqueous solution is Me²⁺ and the ionization of the oxygen-containing groups of GO makes electrostatic attraction to be the main mechanism for the adsorption of heavy metals by GO.^29,30 For the pH = 6–8, hydroxide complex Me(OH)⁺ starts to form.³⁰ Further increasing the pH to 8–12, the main species in the solution gradually become to Me(OH)⁰₂, Me(OH)₃⁻, Me(OH)₄²⁻, etc., and the precipitation of Me(OH)_2(s) coupled with adsorption of other metal hydroxyl complex ions may occur. Similar phenomenon was also observed by Lu et al. in the adsorption of zinc ion with purified carbon nanotube.⁵⁶

To reduce the measurement error brought by metal hydroxide precipitations, the effect of pH was investigated in acidic pH rang of 2–6 with CG8. As shown in Fig. 7f, the adsorption capacities of the four metal ions are highly affected by the solution pH, and exhibit an increasing tendency with pH increasing. This result is closely related to the ionization degree of carboxyl groups in GO and CMCNa, which consequently may exert a significant influence on the complicated interactions between the metal ions and CG8.

4. Conclusions

In summary, CGs were prepared simply in this work via a spray drying method. Meanwhile, hydrogen bonds were formed among the oxygen-containing groups in GO sheets and CMCNa matrix, which enhanced not only the mechanical strength of the microparticles, but also their swelling properties. Moreover, the introduction of GO also significantly improved the adsorption performances of CGs towards dyes and heavy metals. With the addition of 8 wt% GO, the adsorption capacities of the sample increased by 340–354% for dyes and 350–500% for heavy metal ions. Thanks to the microscale structure, CGs exhibited high adsorption rates and the adsorption equilibrium was obtained for all pollutants within 30 min. Further analysis on the adsorption isotherms and kinetics revealed that the adsorption of the pollutants can be well described by the Langmuir isotherm model and the pseudo-second-order model, respectively. The adsorption mechanism for dyes includes both electrostatic and π–π interactions, while for heavy metals, the mechanism consists of electrostatic interactions, surface complexation and ion exchange. Additionally, the adsorption behaviors of our CGs can be tuned easily by the content of GO as well as the solution pH.

Acknowledgements

The work was supported by the Henan Province University Innovation Talents of Science and Technology Support Program (2012HASTIT017), the Science and Technology Department of Henan Province (142300410011, 102102210131 and 152102110073), the Education Department of Henan Province (2010A430002), and Henan University of Technology (2012JCYJ07).

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